GTX 970 - Blender278 - CUDA Error
Searching a little bit more and more, I find many things about Mem Cycles consume and real consume. With GPU-Z I can see a real read.
First of all, the modifiers turned on in render, Subdiv mainly. When blender Mem says 1670Mb, is the limit for my 4Gb GTX 970.
All textures is better to be JPG, PNG. Only use float as EXR or HDRI on particular cases like environment lighting.

Maybe, one day, Cycles developers would think using GPU only to process and hardware RAM to cache everything, like Redshift and Octane do :)